Optical communications systems occupy an important position in communications transmission networks for their huge transmission bandwidths, huge capacity expansion potentials, very low transmission loss and low costs. As the development of high-speed digital-to-analog conversion (DAC) chips, analog-to-digital conversion (ADC) chips and digital signal processing (DSP) chips, the digital coherent optical communications technology becomes a main stream tendency in next generation communications.
In order to construct an optical communications network of low cost, large capacity and high spectral efficiency, it is crucial to select a suitable modulation format. As a most efficient way, a flexible variable modulation format becomes a focused hot spot. However, a complex modulation format is particularly sensitive to a laser phase noise, which results in a severe bit error, thereby greatly limiting a transmission efficiency of the system. Hence, in a typical digital coherent receiver, it is an indispensable operation to perform carrier phase recovery on received signals. Furthermore, in practical application, it is often expected to use a non-feedback blind phase estimation method, to avoid transmission costs caused by feedback delay and training sequences, thereby improving an information transmission efficiency.
In conventional carrier phase recovery (CPR) methods, a commonly-used method is a Viterbi-Viterbi algorithm. A principle of such a method is to remove information on modulation phase items after a biquadratic operation is performed on received signals, to obtain information containing a noise item only, and then its angle is taken and divided by four, to obtain a phase noise estimation value. Another commonly-used method is a blind phase search (BPS) algorithm. A basic principle of such a method is to rotate a received complex signal by n test angles, and then calculate offset distances between the signal and its judgment position in turn, to obtain a minimum offset distance through comparison.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the this disclosure.